The Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS) is a NASA sounding rocket mission that will obtain ultra-violet spectra of comet ISON as it nears the sun. The ultra-violet region contains particularly rich information about comet chemistry. In a suborbital flight that lasts only minutes, FORTIS can observe the comet much closer to the sun than other orbiting observatories.

FORTIS will launch from White Sands Missile Range in New Mexico, and will soar up above 60 miles high, where it will be above much of Earth's atmosphere, which would interfere with the detection of cometary emissions at far-ultraviolet wavelengths.

FORTIS will be able to look at the molecules and atoms inside the comet by taking advantage of the fact that, as the comet moves toward the sun and warms, its rate of sublimation (a process similar to evaporation in which solid matter transitions directly into gas) increases. FORTIS will observe that gas with an instrument called a spectrograph, which can separate incoming light into its component wavelengths. The intensity of light at each wavelength, in turn, corresponds to the sublimation rate of certain atoms. A series of shutters open and close throughout flight to allow the instrument to gather information from several areas at once: different parts of a galaxy, for example, or in this case different parts of the comet.

FORTIS is flying an autonomous targeting system that will locate the center of the comet on the fly and then deploy a pre-programed array of the instrument's microshutters, in order to observer 43 different locations in the comet. Overall, FORTIS will sample a region approximately 625,000 miles in diameter. This system, designed by former Johns Hopkins University graduate student Brian Fleming, uses a prototype of the microshutter arrays built by NASA's Goddard Space Flight Center in Greenbelt, Md. for the Near Infrared Spectrograph on the James Webb Space Telescope. When the instrument comes back down to the ground, the team will recover it and should be able to use it again.

The window for the FORTIS launch begins on 19 Nov. 2013, just nine days before comet ISON's close encounter to the sun. If the comet survives, it will to travel back out into space, unlikely to ever return to the inner solar system.